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sched/core: Convert task_struct.usage to refcount_t 

atomic_t variables are currently used to implement reference
counters with the following properties:

 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable task_struct.usage is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

** Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.

The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.

Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.

Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the task_struct.usage it might make a difference
in following places:

 - put_task_struct(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and control dependency on success
   vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: viro@zeniv.linux.org.uk
Link: https://lkml.kernel.org/r/1547814450-18902-5-git-send-email-elena.reshetova@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Elena Reshetova 2019-01-18 14:27:29 +02:00 committed by Ingo Molnar
parent c45a779524
commit ec1d281923
4 changed files with 7 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/sched.h
View File

@ -21,6 +21,7 @@
#include <linux/seccomp.h>
#include <linux/nodemask.h>
#include <linux/rcupdate.h>
#include <linux/refcount.h>
#include <linux/resource.h>
#include <linux/latencytop.h>
#include <linux/sched/prio.h>
@ -607,7 +608,7 @@ struct task_struct {
randomized_struct_fields_start
void *stack;
atomic_t usage;
refcount_t usage;
/* Per task flags (PF_*), defined further below: */
unsigned int flags;
unsigned int ptrace;

4
include/linux/sched/task.h
View File

@ -88,13 +88,13 @@ extern void sched_exec(void);
#define sched_exec() {}
#endif
#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)
#define get_task_struct(tsk) do { refcount_inc(&(tsk)->usage); } while(0)
extern void __put_task_struct(struct task_struct *t);
static inline void put_task_struct(struct task_struct *t)
{
if (atomic_dec_and_test(&t->usage))
if (refcount_dec_and_test(&t->usage))
__put_task_struct(t);
}

2
init/init_task.c
View File

@ -65,7 +65,7 @@ struct task_struct init_task
#endif
.state = 0,
.stack = init_stack,
.usage = ATOMIC_INIT(2),
.usage = REFCOUNT_INIT(2),
.flags = PF_KTHREAD,
.prio = MAX_PRIO - 20,
.static_prio = MAX_PRIO - 20,

4
kernel/fork.c
View File

@ -717,7 +717,7 @@ static inline void put_signal_struct(struct signal_struct *sig)
void __put_task_struct(struct task_struct *tsk)
{
WARN_ON(!tsk->exit_state);
WARN_ON(atomic_read(&tsk->usage));
WARN_ON(refcount_read(&tsk->usage));
WARN_ON(tsk == current);
cgroup_free(tsk);
@ -896,7 +896,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
* One for us, one for whoever does the "release_task()" (usually
* parent)
*/
atomic_set(&tsk->usage, 2);
refcount_set(&tsk->usage, 2);
#ifdef CONFIG_BLK_DEV_IO_TRACE
tsk->btrace_seq = 0;
#endif